// 定义requestAnimFrame函数
window.requestAnimFrame = function() {
    return (
        window.requestAnimationFrame ||
        window.webkitRequestAnimationFrame ||
        window.mozRequestAnimationFrame ||
        window.oRequestAnimationFrame ||
        window.msRequestAnimationFrame ||
        function(callback) {
            window.setTimeout(callback)
        }
    )
}()

// 初始化函数
function init(elemid) {
    let canvas = document.getElementById(elemid)
    c = canvas.getContext('2d')
    w = (canvas.width = window.innerWidth)
    h = (canvas.height = window.innerHeight)
    c.fillStyle = "rgba(30,30,30,1)"
    c.fillRect(0, 0, w, h)
    return { c: c, canvas: canvas }
}

window.onload = function() {
    let c = init("canvas").c,
        canvas = init("canvas").canvas,
        w = (canvas.width = window.innerWidth),
        h = (canvas.height = window.innerHeight),
        mouse = { x: false, y: false },
        last_mouse = {}

    function dist(p1x, p1y, p2x, p2y) {
        return Math.sqrt(Math.pow(p2x - p1x, 2) + Math.pow(p2y - p1y, 2))
    }

    class segment {
        constructor(parent, l, a, first) {
            this.first = first
            if (first) {
                this.pos = { x: parent.x, y: parent.y }
            } else {
                this.pos = { x: parent.nextPos.x, y: parent.nextPos.y }
            }
            this.l = l
            this.ang = a
            this.nextPos = {
                x: this.pos.x + this.l * Math.cos(this.ang),
                y: this.pos.y + this.l * Math.sin(this.ang)
            }
        }

        update(t) {
            this.ang = Math.atan2(t.y - this.pos.y, t.x - this.pos.x)
            this.pos.x = t.x + this.l * Math.cos(this.ang - Math.PI)
            this.pos.y = t.y + this.l * Math.sin(this.ang - Math.PI)
            this.nextPos.x = this.pos.x + this.l * Math.cos(this.ang)
            this.nextPos.y = this.pos.y + this.l * Math.sin(this.ang)
        }

        fallback(t) {
            this.pos.x = t.x
            this.pos.y = t.y
            this.nextPos.x = this.pos.x + this.l * Math.cos(this.ang)
            this.nextPos.y = this.pos.y + this.l * Math.sin(this.ang)
        }

        show() {
            c.lineTo(this.nextPos.x, this.nextPos.y)
        }
    }

    class tentacle {
        constructor(x, y, l, n, a) {
            this.x = x
            this.y = y
            this.l = l
            this.n = n
            this.t = {}
            this.rand = Math.random()
            this.segments = [new segment(this, this.l / this.n, 0, true)]
            for (let i = 1; i < this.n; i++) {
                this.segments.push(new segment(this.segments[i - 1], this.l / this.n, 0, false))
            }
        }

        move(last_target, target) {
            this.angle = Math.atan2(target.y - this.y, target.x - this.x)
            this.dt = dist(last_target.x, last_target.y, target.x, target.y)
            this.t = {
                x: target.x - 0.8 * this.dt * Math.cos(this.angle),
                y: target.y - 0.8 * this.dt * Math.sin(this.angle)
            }
            if (this.t.x) {
                this.segments[this.n - 1].update(this.t)
            } else {
                this.segments[this.n - 1].update(target)
            }
            for (let i = this.n - 2; i >= 0; i--) {
                this.segments[i].update(this.segments[i + 1].pos)
            }
            if (dist(this.x, this.y, target.x, target.y) <= this.l + dist(last_target.x, last_target.y, target.x, target.y)) {
                this.segments[0].fallback({ x: this.x, y: this.y })
                for (let i = 1; i < this.n; i++) {
                    this.segments[i].fallback(this.segments[i - 1].nextPos)
                }
            }
        }

        show(target) {
            if (dist(this.x, this.y, target.x, target.y) <= this.l) {
                c.globalCompositeOperation = "lighter"
                c.beginPath()
                c.moveTo(this.x, this.y)
                for (let i = 0; i < this.n; i++) {
                    this.segments[i].show()
                }
                c.strokeStyle = "hsl(" + (this.rand * 60 + 180) + ",100%," + (this.rand * 60 + 25) + "%)"
                c.lineWidth = this.rand * 2
                c.lineCap = "round"
                c.lineJoin = "round"
                c.stroke()
                c.globalCompositeOperation = "source-over"
            }
        }

        show2(target) {
            c.beginPath()
            if (dist(this.x, this.y, target.x, target.y) <= this.l) {
                c.arc(this.x, this.y, 2 * this.rand + 1, 0, 2 * Math.PI)
                c.fillStyle = "whith"
            } else {
                c.arc(this.x, this.y, this.rand * 2, 0, 2 * Math.PI)
                c.fillStyle = "darkcyan"
            }
            c.fill()
        }
    }

    let maxl = 400,
        minl = 50,
        n = 30,
        numt = 600,
        tent = [],
        clicked = false,
        target = { x: 0, y: 0 },
        last_target = {},
        t = 0,
        q = 10

    for (let i = 0; i < numt; i++) {
        tent.push(new tentacle(
            Math.random() * w,
            Math.random() * h,
            Math.random() * (maxl - minl) + minl,
            n,
            Math.random() * 2 * Math.PI
        ))
    }

    function draw() {
        if (mouse.x) {
            target.errx = mouse.x - target.x
            target.erry = mouse.y - target.y
        } else {
            target.errx = w / 2 + ((h / 2 - q) * Math.sqrt(2) * Math.cos(t)) / (Math.pow(Math.sin(t), 2) + 1) - target.x
            target.erry = h / 2 + ((h / 2 - q) * Math.sqrt(2) * Math.cos(t) * Math.sin(t)) / (Math.pow(Math.sin(t), 2) + 1) - target.y
        }
        target.x += target.errx / 10
        target.y += target.erry / 10
        t += 0.01

        c.beginPath()
        c.arc(target.x, target.y, dist(last_target.x, last_target.y, target.x, target.y) + 5, 0, 2 * Math.PI)
        c.fillStyle = "hsl(210,100%,80%)"
        c.fill()

        for (i = 0; i < numt; i++) {
            tent[i].move(last_target, target)
            tent[i].show2(target)
        }

        for (i = 0; i < numt; i++) {
            tent[i].show(target)
        }

        last_target.x = target.x
        last_target.y = target.y
    }

    function loop() {
        window.requestAnimFrame(loop)
        c.clearRect(0, 0, w, h)
        draw()
    }

    window.addEventListener("resize", function() {
        w = canvas.width = window.innerWidth
        w = canvas.height = window.innerHeight
        loop()
    })

    loop()
    setInterval(loop, 1000 / 60)

    canvas.addEventListener("mousemove", function(e) {
        last_mouse.x = mouse.x
        last_mouse.y = mouse.y
        mouse.x = e.pageX - this.offsetLeft
        mouse.y = e.pageY - this.offsetTop
    }, false)

    canvas.addEventListener("mouseleave", function(e) {
        mouse.x = false
        mouse.y = false
    })
}